/Engine/Plugins/Online/OnlineSubsystem/Source/Public/Interfaces/OnlinePartyInterface.h
Returns the number of players in a given party
uint32 GetPartyMemberCount ( const [FUniqueNetId](API\Runtime\CoreOnline\Online\FUniqueNetId) & LocalUserId, const [FOnlinePartyId](API\Plugins\OnlineSubsystem\FOnlinePartyId) & PartyId ) const